This invention is a further advance pertaining to the structure of and the method for fabricating controlled porosity dispenser cathodes.
Thermionic emission cathodes of the type that can be used in microwave tubes such as travelling wave tubes are, in many applications, required to have high reliability and long life. It is also desirable that all areas of the cathode surface be operated in a space charge limited mode for more stable operation.
Conventional cathodes, such as the "B", the "S" and the "M" types, are made of random porosity structures impregnated with barium calcium aluminate compounds and generally tend to provide a non-uniform emission over the surface of the cathode. The result is that excessive temperatures are required to assure that space charge limited emission is achieved in less active areas of the cathode surface. Consequently, the more active areas become excessively hot resulting in decreased reliability and shortened life. The end result is that emission is gradually reduced as the pores become depleted of the impregnant. Examples of these types of cathodes are disclosed in U.S. Pat. No. 2,700,000, issued on Jan. 18, 1955 to R. Levi et al and U.S. Pat. No. 2,722,626, issued on Nov. 1, 1955 to P. P. Coppola et al.
In order to attain the goals of long life and reliability, a number of approaches have heretofore been used. Previously, a thin layer of porous metal was formed directly on the emitting surface of the reservoir of activating material such as by evaporation in a vacuum, by electroplating or by vapor deposition. These approaches are disclosed in U.S. Pat. No. 3,155,864, issued to P. P. Coppola on Nov. 3, 1964 and in U.S. Pat. Nos. 3,243,637 and 3,243,638 issued to J. H. Affleck III on Mar. 29, 1966.
Shortcomings in these approaches are that the porosity of the emitter surface is random rather than precise and coating directly to the activating material could block many of the pores.
One of these more recent approaches is disclosed in U.S. Pat. No. 4,101,800, issued on July 18, 1978 to R. E. Thomas wherein a reservoir of activating material is covered by a perforated metal foil. The perforations enable migration of electron emitting material from the reservoir of activating material to the foil surface to coat the surface thereby providing a cathode surface of somewhat uniform emissivity.
Subsequently, an advance was made in the fabrication of such structures as disclosed in U.S. Pat. No. 4,310,603, issued on Jan. 12, 1982 to L. R. Falce. In this approach, a perforated metal foil having an appropriate pattern of pore size apertures therein is formed. Thereafter, this foil is bonded to a generally cylindrical housing such as by brazing, welding or diffusion bonding.
Several disadvantages of this last approach are that: the high temperatures associated with the bonding process cause recrystallization of the foil material; the use of dissimilar brazing materials subject the foil surface to contaminates whereupon the foil has a non-uniform work function; and the braze material can block some of the apertures in the emitter surface. On top of this, the brazing or welding with unlike materials creates a distinct possibility that the bond will fail during thermal cycling. Moreover, fabrication of this kind of device requires a large number of hand processing steps.